Lithium ion secondary batteries generally include: a positive electrode comprising a positive electrode substrate and a positive electrode material mixture layer carried thereon; a negative electrode comprising a negative electrode substrate and a negative electrode material mixture layer carried thereon; a separator; and a non-aqueous electrolyte. The electrode material mixture layer comprises an active material and an electrode binder, and the electrode binder is usually a thermoplastic resin, a modified rubber material, or the like. Specifically, first, an electrode mixture slurry is prepared by mixing an active material, an electrode binder, and a dispersion medium of the active material. The electrode mixture slurry is applied onto an electrode substrate, dried with hot air at 130° C. or lower, and roll pressed, to form an electrode material mixture layer (see Patent Document 1).
The separator interposed between the positive electrode and the negative electrode has functions of electronically insulating the electrode plates and retaining the electrolyte. The separator is mainly a microporous sheet composed of polyethylene resin. However, such a separator sheet as a microporous sheet is generally subject to shrinking even at temperatures of 150° C. or lower, thereby causing the battery to short-circuit. Also, when a sharp object such as a nail penetrates the battery (for example, upon nail penetration test), heat is instantaneously produced due to a short-circuit reaction, thereby causing the microporous sheet to shrink, which results in expansion of the short-circuit.
Recently, in order to improve quality, it has been proposed to bond a porous film, made of a particulate filler and a film binder, to the surface of an electrode in order to use the electrode integrated with the porous film. In this case, first, a porous film paste is prepared by mixing a particulate filler, a film binder, and a dispersion medium of the particulate filler. The porous film paste is applied onto the electrode surface and dried with hot air (see Patent Document 2).
The film binder that has been conventionally used is a resin that dissolves or disperses in the above-mentioned dispersion medium. The applied film of the porous film paste is dried with hot air to evaporate the dispersion medium, in order to form a porous film. However, such a porous film obtained in the above manner has poor strength, and swelling with an electrolyte and dissolution into the electrolyte of the film binder tend to occur. Also, such a porous film may separate from the electrode surface when it is subjected to stress exerted by expansion and contraction of the electrode active material. These phenomena are believed to cause degradation of life characteristics of lithium ion secondary batteries.
Patent Document 1: Japanese Laid-Open Patent Publication No. Hei 10-334877
Patent Document 2: Japanese Laid-Open Patent Publication No. Hei 7-220759